1. Technical Field
The present invention relates to electronic compasses and more specifically to a method for removing noise from data in an electronic compass system.
2. Discussion
Normally electronic compass systems employ a magnetic flux-gate sensor. An example of an electronic compass system can be found in U.S. Pat. No. 4,622,843 to Hormel, issued Nov. 18, 1986, entitled "Simplified Calibration Technique and Auto-Ranging Circuit for an Electronic Compass Control Circuit." This reference is hereby incorporated by reference.
The operation of the flux-gate sensor is well documented. See, "Magnetic Field Sensor and its Application to Automobiles", SAE Paper 800123, pages 83-90, February, 1980, by Hisatsugu Itoh and "A Magnetic Heading Reference for the Electro Fluidic Autopilot", Sport Aviation by Doug Garner, Part I, pages 19-26, November, 1981 and Part II, pages 20-32, 51, December, 1981. The disclosures of these documents are hereby incorporated by reference.
A method for calculating headings is found in U.S. Pat. No. 4,807,462 issued Feb. 28, 1989, to Al-Attar entitled "Method for Performing Automatic Calibration in an Electronic Compass." This patent is also hereby incorporated by reference.
Signals from flux-gate sensors are often less than clean. Compass accuracy is plagued by rapid changes in field strength. For example, a vehicle passing local magnetic anomalies at high speed causes noise signals. Transients and other time variant magnetic fields also create noise.
This noise manifests itself as Jitter in the compass reading. When compass data is displayed in degrees, the heading fluctuates in small degree increments. When compass data is displayed as a compass point, for example, "northwest", jitter can cause a display displaying such information to vary greatly in brightness.